nixos/cosmic-greeter: allow empty passwords

Auto-login masked part of this issue because the greeter was never
interacted with. But once the user's session gets locke, it never
unlocks because PAM doesn't allow empty passwords. This patch fixes that
behaviour.

nixos/modules/services/display-managers/cosmic-greeter.nix

@@ -73,7 +73,9 @@ in
       extraGroups = [ "video" ];
     };
     # Required for authentication
-    security.pam.services.cosmic-greeter = { };
+    security.pam.services.cosmic-greeter = {
+      allowNullPassword = true;
+    };
 
     hardware.graphics.enable = true;
     services.accounts-daemon.enable = true;